An anti-government protestor reacts as he and other demonstrators shout slogans during a demonstration demanding the resignation of Yemeni President Ali Abdullah Saleh, in Sanaa, Yemen, Tuesday, March 1, 2011. Yemen's embattled president has accused the United States and Israel of trying to destabilize his country and the Arab world.

Yemen’s embattled president on Tuesday accused the U.S., his closest ally, of instigating the mounting protests against him, but the gambit failed to slow the momentum for his ouster.

Hundreds of thousands rallied in cities across Yemen against the government of President Ali Abdullah Saleh, in the largest of the protests of the past month, including one addressed by an influential firebrand cleric, a former ally of Saleh, whom the U.S. has linked to al-Qaida.

“Go on until you achieve your demands,” Sheik Abdul-Majid al-Zindani told tens of thousands of demonstrators in the capital of Sanaa. A former U.S. ambassador to Yemen called al-Zindani’s decision to turn against President Ali Abdullah Saleh a major setback for the president.

Some warned that the current political turmoil and possible collapse of Saleh’s regime could give a further opening to Yemen’s offshoot of the global terror network, al-Qaida in the Arabian Peninsula.

James Jones, former White House National Security Advisor, warned a Washington conference that Yemen’s crisis “could deepen the current vacuum of power in Yemen on which al Qaida has thrived.”

The Yemen-based al-Qaida in the Arabian Peninsula, believed to have been involved in the attempted 2009 bombing of an American airliner, is seen as particularly active and threatening to the U.S.

Saleh has been a weak but important U.S. ally in the fight against al-Qaida, accepting tens of millions of dollars in U.S. military and other aid and allowing American drone strikes on al-Qaida targets.

Garry Reid, deputy assistant U.S. Secretary of Defense for Special Operations and Combating Terrorism, told the Bipartisan Policy Center, a Washington think tank, that the Saleh government was “the best partner we’re going to have … and hopefully it will survive because I certainly would have to start over again in what we’ve tried to build.”

Secretary of State Hillary Rodham Clinton visited Yemen in January and urged Saleh to do more.

However, on Tuesday, Saleh seemed to be turning on Washington. In a speech to about 500 students and lecturers at Sanaa University, he claimed the U.S., along with Israel, is behind the protest movement.

“I am going to reveal a secret,” he said. “There is an operations room in Tel Aviv with the aim of destabilizing the Arab world. The operations room is in Tel Aviv and run by the White House.”

Saleh also alleged that opposition figures meet regularly with the U.S. ambassador in Sanaa. “Regrettably those (opposition figures) are sitting day and night with the American ambassador where they hand him reports and he gives them instructions,” Saleh said.

The Obama administration rejected these claims. White House spokesman Jay Carney called on Saleh to focus on implementing the political reforms demanded by his people instead of “scapegoating.”

Saleh’s relationship with the U.S. has been ambivalent, and he has at times attempted to play down his military alliance with Washington. Anti-U.S. sentiment remains strong in Yemen, as elsewhere in the region, and Saleh’s comments appeared to be an attempt to discredit the protesters by suggesting they are serving foreign interests.

“Part of this is putting blame on others, part of it is trying to manage the situation,” said Christopher Boucek, an analyst at the Carnegie Endowment for International Peace, a U.S. think tank. “He (Saleh) does not want to feed into grievances that gave rise to the opposition against him, such as being too close to the U.S.”

Thomas Krajeski, senior vice president of the U.S. National Defense University and former U.S. ambassador to Yemen, gave the Saleh regime a 50-50 chance of surviving the current crisis and he said it’s not clear who is likely to succeed him. “We just don’t know what comes next,” Krajeski told a conference at Washington’s Bipartisan Policy Center.

But Krajeski predicted that Yemen’s tribes would quickly step in to establish a new government rather than let the country become what he called “an ungoverned mess,” like Somalia.

Jonathan Ruhe, a policy analyst for the Bipartisan Policy Center, said: “It’s kind of hard to imagine a post-Saleh world. If he should fall, the future is wide open.”

In another attempt to silence critics, Saleh fired five of the country’s 22 provincial governors Tuesday, including three who had spoken out against the government’s at times violent crackdown on demonstrators.

In London, Britain’s Foreign Office summoned a senior Yemeni diplomat to express “deep concern” over the deaths of protesters at rallies. “The government of Yemen should listen to the legitimate grievances of the Yemeni people,” the Foreign Office said.

The momentum against the president, who refuses to step down until elections in 2013, has kept growing since protests erupted a month ago — inspired by successful uprisings in Egypt and Tunisia. He has lost the support of key tribal chiefs and on Tuesday, opposition parties called their supporters into the streets for the first time. Crowds of tens of thousands each were reported in five areas of the country, including in Sanaa.

Saleh’s government is widely seen as corrupt, with relatives of the president holding key positions in government and business. Grievances about the growing disparity between Yemen’s poor — nearly half the population of some 23 million — and a small ruling clique have helped drive the protests. Yemen is the Arab world’s poorest country.

In the port city of Aden, the scene of deadly clashes between police and demonstrators last week, thousands rallied Tuesday to express their anger. “We are demonstrating and calling for the downfall of the regime because Aden, under Saleh, has turned into a village,” said Faiza al-Sharbary, a 45-year-old teacher. “At one time, it was one of the best cities. Therefore this regime has to leave.”

In Sanaa, tens of thousands gathered outside the university, the heart of the protests.

Al-Zindani, the influential Islamic cleric, praised the young protesters, saying their rallies are “a new way to change regimes that we did not know 50 years ago.”

“Go on until you achieve your demands,” he told them. “You have come out demanding changes as a result of desperation.”

Al-Zindani’s role appeared unclear. Saleh, in power for 32 years, has tried to co-opt the preacher, appointing him last year as a mediator between the government and opposition parties over electoral reform.

However, al-Zindani is also thought by the United States to be a one-time spiritual mentor of Osama bin Laden. He has been placed on the U.S. list of terrorist financiers, and is the subject of travel and financial sanctions by the U.S. and the United Nations.

In the past, the cleric has criticized the U.S.-backed fight against al-Qaida, warning that it could lead to a foreign occupation of Yemen.

Some in Yemen said the current turmoil could strengthen the local al-Qaida branch.

“One of the principal worries of our regional and global partners has been that if Yemen goes into anarchy, the possibility of al-Qaida having easy access should be quite clear,” said Mohamed Qubaty, a senior member of Yemen’s ruling party.

Krajeski, the former U.S. ambassador to Yemen, said al-Zidani’s decision to criticize Saleh and questioned his legitimacy was a major setback for the government. “That’s a big deal,” he said. “Saleh worked very hard to keep this guy in control. If Zindani is breaking with him that is another knock on his base.”

Krajeski added though that he didn’t think that radical Islam was a big factor in the current unrest, although it was part of the general opposition to Saleh’s government.

Yemen has been the site of numerous anti-U.S. attacks, going back to the 2000 bombing of the USS Cole in Aden harbor, which killed 17 American sailors. Late last year, several CIA operatives were targeted in a failed bombing at a restaurant in a Sanaa suburb. Al-Qaida in the Arabian Peninsula was also thought to be behind the attempted bombing of an American airliner landing in Detroit in 2009.